1. Field of the Invention
The present invention relates to the improvement of a technology for obtaining a metallic iron by reducing an iron oxide such as an iron ore with a carbonaceous reductant such as a carbonaceous material by heat. More specifically, the invention relates to an efficient production method of a high purity metallic iron as a molten iron by efficiently reducing the iron oxide to the metallic iron at the time an iron oxide such as an iron ore is reduced with a carbonaceous reductant such as a carbonaceous material by heat to obtain a metallic iron as well as by melting and separating slag components contained in an iron oxide source such as an iron ore as gangue components.
2. Description of the Related Art
As a direct iron-producing method where a reduced iron is obtained by directly reducing an iron oxide such as an iron ore or iron oxide pellets with a carbonaceous material or a reducing gas, a shaft furnace method represented by the Midrex process has been known conventionally. The direct iron-producing method is a method where a reduced iron is obtained by blowing a reducing gas produced from a natural gas or the like into a shaft furnace from a tuyere provided at the bottom thereof so as to utilize the reducing ability for reducing the iron oxide. In recent years, a production method of a reduced iron where a carbonaceous material such as coal is used as a reductant in place of a natural gas has attracted attention. Specifically, the so-called SL/RN method where sintered pellets produced from an iron ore are reduced with coal powders by applying heat in a rotary kiln has already been put into practice.
Another production method of a reduced iron is disclosed in the official gazette of the U.S. Pat. No. 3,443,931 where a carbonaceous material and a powdery iron oxide are mixed to form agglomerates, and heated on a rotary hearth for reduction. The process comprises the steps of mixing a powdery iron ore and a powdery carbon to form agglomerates, and reducing them in a high temperature atmosphere by heat.
A reduced iron produced in the above-mentioned methods is charged into an electric furnace directly or after being prepared as briquettes so as to be used as an iron source. With the recent active movement to recycling iron scrap, a reduced iron obtained in the above-mentioned methods has drawn attention as a diluent of impurities contained in the scrap.
However, since slag components such as SiO2, Al2O3, CaO contained in the iron oxide such as an iron ore, or in the coal material such as coal used as a material are introduced in an iron oxide obtained in the conventional production method of a reduced iron, the iron content of a product (iron purity of metallic iron) is low. In actual practice, the slag components are separated and eliminated in a subsequent refining process. However, since an increase in the amount of slag not only decreases the yield of refined molten iron, but also has a great influence on the running cost of an electric furnace, an iron-rich reduced iron with a low content of slag components is required. In order to meet the requirement, it is necessary to use an iron-rich iron ore as a material for producing a reduced iron in the above-mentioned conventional production methods of a reduced iron, which narrows the choice of materials for producing iron.
Furthermore, a goal of the above-mentioned conventional methods is to obtain a reduced solid product, as an intermediate product so that additional steps including transportation, storage, briquette formation, and cooling are required before a subsequent refining process. This is disadvantageous since a large energy loss occurs during the steps, and it requires extra energy and a special apparatus for briquetting.
On the other hand, a melting reduction process where an iron oxide is directly reduced to obtain a reduced iron such as the DIOS method is known. In this method, an iron oxide is preliminarily reduced to an iron purity of 30 to 50%, then reduced to a metallic iron by the direct reducing reaction with carbon in an iron bath. However, this method involves problems in that the need of the two steps including the preliminary reduction and the final reduction in the iron bath complicates the operation, and direct contact of a molten iron oxide (FeO) in an iron bath and a refractory causes the significant damage of a refractory in a furnace.
Furthermore, Japanese Examined Patent Publication No. 56-19366 discloses a method where lumps containing a metal oxide, a solid carbonaceous material, and a slag forming material are reduced by heat, a metal generated by the reduction is contained by a slag shell, then the metal and the slag are separated by melting the slag shell. However, a slag sufficient for completely containing the method needs to be produced for preventing the re-oxidization of a metal generated by the reduction in the method. Otherwise with an insufficient content of a slag forming material, the consequent insufficiency in containing the metal results in inevitable re-oxidization of the metal. Besides, a large problem is involved in practice in that a slag with a high FeO concentration can be produced depending on a heat-reduction condition so as to remarkably damage the interior refractory of the equipment.
As heretofore mentioned, realization of a production method of a metallic iron with a small slag component content is extremely important in view of not only increasing the added value of a metallic iron as a product, but also reduction of iron-producing cost using an electric furnace, and flexibility in choosing the materials in producing a metallic iron. Furthermore, it is also significantly important to minimize the iron oxide content in a slag produced in the heating and reducing process as a by-product so as to restrain the damage of the refractory for realizing the iron-producing method industrially.
Japanese Unexamined Patent Publication No. 7-54030 discloses a method for producing steel using sponge iron, partially-reduced iron, self-reducing pellets, or fine iron ore as an iron source as an integrated steel production method to take the place of a blast furnace-converter method, although it is a method belonging to a field different from that of the present invention. That is, the method produces steel by introducing the above-mentioned iron source, in particular a material with a high iron content, into a channel type induction furnace, and maintaining the temperature in the furnace at not lower than the liquidus curve temperature of the product by controlling the quantity of heat supplied to the furnace and the introduction rate of the iron source. It is mentioned that effects can be achieved according to this method in that steel with a carbon content of about 0.1% by weight can be produced continuously in an induction furnace instead of the conventional iron and steel production using both a blast furnace and a converter so that equipment and processes can be simplified, and the energy efficiency can be improved.
However, since reduction with carbon of an iron oxide present in an unreduced iron source, and elimination of carbon taken into the molten iron by reduction (oxidization), should be conducted in the same furnace, it is extremely difficult to control the supply amount of a carbonaceous material or oxygen and the processing temperature. Further, damage to the refractory wall inside the furnace is significant due to existence of iron oxide in a large amount in molten slag generated in a large amount, so that many problems can be expected in terms of both equipment and operation in using this method industrially.
The present inventors paid attention to the situation and studied for developing a technology capable of efficiently obtaining a metallic iron with an extremely high iron purity as a molten iron with a simple processing, even from an iron ore with a relatively low content of an iron component, without the risk of damaging a refractory. As a result, the following method has been developed and disclosed in Japanese Unexamined Patent Publication No. 8-59801.
The prior technology where a metallic iron is produced by reducing an iron oxide compacted with a carbonaceous reductant by heat has the following aspects:
(1) A shell containing a metallic iron is generated and grown by reduction by heat. The reduction is continued until substantially no iron oxide remains in the shell, and agglomerates of generated slag are generated in the shell. PA1 (2) A shell containing a metallic iron is generated and grown by reduction by heat. The reduction is continued until substantially no iron oxide remains in the shell. Heat application is further continued such that slag generated in the shell is discharged outside the metallic iron shell. PA1 (3) A shell containing a metallic iron is generated and grown by reduction by heat. The reduction is continued until substantially no iron oxide remains in the shell. Heat application is further continued such that molten metallic iron and molten slag are separated. PA1 (4) A shell containing a metallic iron is generated and grown by reduction by heat. The reduction is continued until substantially no iron oxide remains in the shell as well as generated slag is agglomerated in the shell, followed by a process of separating the generated slag from the metallic iron.
In order to embody the above-mentioned method (2), molten slag in the shell can be discharged outside the metallic iron shell by partially melting the metallic iron shell. In this case or in order to embody the above-mentioned method (3), carburization may be continued with a carbonaceous reductant present in the metallic iron shell so as to lower the melting point of the metallic iron shell so that a part or the entirety of the metallic iron shell can be melted.
In embodying any of the above-mentioned methods (1) to (4), the reaction of generating a metallic iron can be conducted more efficiently by controlling the maximum heating temperature in the heat reduction process to be not less than the melting point of the generated slag and not more than the melting point of the metallic iron shell. In this reducing process, the purity of the metallic iron to be obtained can be efficiently improved by reducing the iron oxide by a solid phase reduction, and further reducing the same by a liquid phase reduction until substantially no iron oxide, composed mainly of FeO, is present.
In order to accomplish the reduction of an iron oxide in a solid phase effectively, it is necessary that slag generated in the reduction process is melted at a lower temperature with respect to a metallic iron generated by the reduction. Therefore, it is preferable that the content composition of a slag component contained in the iron oxide or the carbonaceous reductant be compacted preliminarily so that the melting point of the generated slag can be lower than the melting point of the reduced iron by adding Al2O3, SiO3, or CaO in the compacting process as needed.
In the above-mentioned prior technology, the term "reduction is continued until substantially no iron oxide remains in the metallic iron shell" means, on a quantitative basis, "reduction is continued until the content of an iron oxide mainly composed of FeO is 5% by weight or less, preferably 2% by weight or less," in the heat reduction process. From a different point of view, it means that the reduction by heat is continued until the content of an iron oxide mainly composed of FeO in the slag separated from a metallic iron generated in the reducing reaction is preferably 5% by weight or less, more preferably 2% by weight or less.
A metallic iron of an extremely high purity with a metallization ratio of about 95% or higher, or further, of about 98% or higher can be obtained by melting the metallic iron of a high purity and the produced slag obtained in the method so as to be separated by the specific gravity difference. Furthermore, according to the prior invention, since the iron oxide content in the produced slag can be minimized so that damage of the refractory in the furnace derived from the iron oxide can be prevented, the practice of the technology is practical in terms of equipment maintenance.